Boundaries

The isolated islands of the Mascarenes ecoregion host a highly endemic aquatic biota, including several gobies that inhabit swift streams. The islands are of volcanic origin and are situated in a line along a mid-Ocean ridge, located east of Madagascar in the western Indian Ocean. The largest islands are the French Dependent Territory (Département) of Réunion (2,500 km²) and the island of Mauritius (1,900 km²). Mauritius, along with Rodrigues (110 km²) and several smaller islands, form the single independent nation of Mauritius. Réunion, at about 690 km from Madagascar, is the most westerly of the islands, followed by Mauritius; Rodrigues lies some 570 km further to the east. 

Topography

The islands have a rugged topography with many ravines and cliffs. On Réunion, Piton de la Fournaise (2,525 m) becomes active several times each year, and as a result the island’s topography is particularly steep. The other major peak on Réunion is Piton des Neiges, a long extinct volcano that reaches 3,069 m and is the highest peak in the Indian Ocean (Defos du Rau 1960). On the island of Mauritius, the highest point is Piton de la Rivière Noire, which reaches a height of only 828 m. Mauritius is considered to be the oldest of the Mascarene Islands and was formed by three periods of volcanic activity occurring between 10 million and 20,000 years ago (Montaggioni & Nativel 1988). The island consists of an irregular central plateau at about 300-600 m, surrounded by three mountain ranges and plains (Ramdin 1969). Rodrigues has even lower relief, with rolling hills rising only to 390 m. Though comprised primarily of basaltic lava and some volcanic dust, there are small areas in the south and the east of Rodrigues where wind-blown sand has accumulated to form limestone rocks. In these areas caves have formed (Sok Appadu & Nayamuth 1999). Many smaller islands also occur in this ecoregion; of particular interest is Round Island, a small island off the northern tip of Mauritius with a distinctive flora and fauna (especially reptiles) comprising many species found nowhere else.

Freshwater habitats

Fast-flowing rivers and streams are relatively abundant on the Mascarene Islands. Several of these rivers are marked by waterfalls, the highest being Tamarin Falls or Sept Cascades (293 m). Mangroves occur along the coast near river mouths and estuaries. On Mauritius, small areas of mangrove are present at various locations including Terre Rouge, Riviere Noire, Baie du Cap, Riviere du Rempart, Trou d’Eau Douce, Poste Lafayette, Bras d’Eau, Roches Noires, and Poudre d’Or (Institute of Marine Sciences et al. 1998).

Terrestrial habitats

The main communities of native vegetation on the islands historically were probably lower montane wet evergreen forest, scrub and marsh vegetation in the uplands, evergreen or semi-deciduous forest in the rain-shadow and palm savanna at lower elevations (Le Corre & Safford 2001). Habitats ranged from coastal wetlands and swamp forests at low elevations, through lowland dry forest, rain forest, and palm savanna, to montane deciduous forests and finally (on Réunion) to heathland vegetation types on the highest mountains. Most of the original vegetation has been destroyed and almost all remaining native plant communities have been badly degraded by introduced species (Stuart et al. 1990; WWF and IUCN 1994). Major plant families include Sapotaceae, Ebenaceae, Rubiaceae, Myrtaceae, Clusiaceae, Lauraceae, Burseraceae, Euphorbiaceae, Sterculiaceae, Pittosporaceae, and Celastraceae. There is a high diversity of palm species on the islands, including many endemic genera (WWF and IUCN 1994).

Description of endemic fishes

About twenty species of fish are tied to freshwater habitats for part of their life cycle and were included in our richness estimates. Of these, five are endemic to the Mascarenes (Hypseleotris cyprinoides, Cotylopus acutipinnis, Glossogobius kokius, Gobius commersonii, and Oxyurichthys guibei). Cotylopus acutipinnis, a goby endemic to Réunion, hatches in the sea and following its larval stage migrates to swift rainforest streams to live as an adult (FishBase 2001). Sicyopterus lagocephalus, a goby found predominantly in the Mascarenes, has a similar life history. 

Justification for delineation

The Mascarene Islands define the boundaries of this ecoregion, which supports an endemic freshwater fauna that shares affinities with Madagascar, East Africa, and Asia. All of the endemic fish species are from the marine family, Gobiidae.

Level of taxonomic exploration

Poor